Poppy i've done a sig 556 upper with wheeler ceramic coat from midway usa and it matched the factory black sig finish very well. it actually seemed like a better finish than the sig factory finish. it comes in a rattle can so it's easy to apply but it does need to be heated after it's applied. 350 degree's for an hour.
http://www.midwayusa.com/product/455475/wheeler-engineering-cerama-coat-4-oz-aerosol-black
i picked up a 556 from my local fun store that had been traded in. it had alot of swiss parts on it and i wanted the lower for my sbr. whoever built the rifle had duracoated the upper in sig gray and i needed it black to match my er lower. here are a couple of pics of b4 and after.
notice how the sig gray duracoat on the upper is a darker shade of gray than the lower
here's the after with the cerama coat
if you don't mind baking it for an hour i would highly recommend it over just "kryloning" the lower. it has held up extremely well and the color match is damn close. it was funny to see your thread as i've been "kryloning" my sig sbr tonight
here's a pic of it last night after test fitting all the parts